Isomerization of light paraffins is often conducted to increase the octane content of gasoline. Generally, such isomerization processes are conducted on separate light hydrocarbon fractions. As an example, isomerization of butane, or pentane and/or hexane (hereinafter may be abbreviated pentane-hexane) is undertaken in separate isomerization units to improve the gasoline quality. Typically, both the isomerization of butane or pentane-hexane are conducted in a fixed-bed liquid/vapor phase or vapor phase process. The reactor can receive a feed of the light paraffins mixed with a gas including a substantial amount of hydrogen.
In the isomerization of butane or pentane-hexane, water is a poison that can reduce the life expectancy of the reactor catalyst. As such, it is desirable to remove water before the hydrogen rich gas and/or the paraffin feed reaches the reactor. Consequently, typically both the feed and the gas are passed through separate drier units to remove water.
Often, two driers are utilized in either series or parallel with alternating regeneration operations, whether the fluid being processed is a gas rich in hydrogen or a hydrocarbon containing butane or pentane-hexane. As such, one drier can be in operation while the other drier may be regenerating. At the end of the regeneration, the drier can contain a gas regenerant if the drier is a gas drier, or a liquid regenerant if the drier is a hydrocarbon feed drier. Depending on the hydrocarbon fraction being isomerized, the regenerant can include mostly an isomerized product, such as isobutane, or at least one of isopentane and isohexane (hereinafter may be referred to as isopentane-isohexane); or the regenerant can include a mixture of one or more different branched, normal, and cyclic compounds. In either instance, generally the regenerant is flushed out of the drier before or as the regenerated drier enters into service. The regenerant may be removed from the system as a net stream.
The gas regenerant can cause upsets in the downstream vessels. Particularly, the gas regenerant can cause a drop in reaction temperatures as the regenerant replaces the hydrogen used in the reactor, and disrupts the hydrogen:hydrocarbon mole ratio in the reactor. In addition, generally the gas regenerant has a heavier molecular weight than the hydrogen rich gas. As a consequence, replacing the hydrogen rich gas may upset the gas flow controls, such as the make-up gas flow, as well as disturbing the pressure controls in a distillation column, which is typically used downstream of the reactor. Thus, there is a desire to lessen the impact after the regeneration of the gas drier to minimize upsets of the downstream vessels.